1. Field of the Invention
A method of fabricating a heat exchanger of the type including a plurality of tubes extending between the first and second tank headers with a pair of reinforcing members extending along the opposite sides of the tubes and attached to the tank headers to compensate for the differences in thermal stresses between the reinforcing members and the tubes.
2. Description of the Prior Art
Typical automotive heat exchangers, such as radiators, include a plurality of thin-walled tubes interleaved with corrugated fins enclosed in a core frame. The fins are rigidly attached to the tubes as well as to a pair of frame reinforcing members while the tubes are jointed to a pair of headers. The frame reinforcing members are attached also to the headers. As is well known in the art, coolant passes from one header through the tubing to the other header. As the temperature of the coolant passing through the heat exchanger core increases, the core expands. The frame reinforcing members, however, are not in direct heat contact with the liquid and do not heat at a proportional rate to the heating of the tubing. In use, hot fluid passes through the tubes and a passage of air over the tubes and the fins reduces the temperature of the fluid. However, since the overall temperature of the tubes is relatively high, the tubes thermally expand by a substantial amount with respect to their length when cold. In use, coolant heated by the engine of the associated vehicle enters one tank and flows through the core tubes. The high temperature of the fluid causes heat transfer by conduction and connection to the walls of the tube and on to the fins of the radiator. Air passes over the fins and over the outer periphery of the tubes to cool the fluid therein in a known fashion. Typically the tubes may be of aluminum or brass both of which have relatively high coefficients of expansion. Thus the hot water causes the tubes to tend to expand thus increasing the separation between the two headers. However, use of a conventional reinforcing member would substantially maintain the spacing between the two headers, because the reinforcing members are not subjected to the same high temperatures as the tubes. The result of the tendency of the tubes to grow in length, while the reinforcing members grow less, is to place high stresses on the region where the tubes are secured to the tank header wall. As a result of the expansion and contraction of the tubing, the reinforcing members induce thermal stress in the tube-to-header joints during the thermal cycling of the heat exchanger.
To overcome this thermal cycling problem, it is known in the art to relieve the thermally-induced stress by an expansion joint system, as disclosed in U.S. Pat. No. 3,939,908 to Chartet. The expansion of the reinforcing member of the radiator has also been mitigated by saw cutting the reinforcing members following brazing of the core and prior to placing the heat exchanger core into service, as disclosed in U.S. Pat. No. 5,954,123 to Richardson. However, the saw cutting operation is difficult to automate, is excessively loud, and produces a tremendous amount of metal fines resulting in increased downtime and increased maintenance of the saw.
Other methods have been proposed to relieve the thermally-induced stress in the heat exchanger core without the need for saw cutting the side supports. For example, U.S. Pat. No. 4,719,967 proposes the sue of a “T-shaped” or “I-shaped” slot or piercing stamped into the core reinforcement prior to forming the reinforcement into a channel member. After brazing the core assembly, the reinforcement is fractured at the perforation to allow for expansion of the core during thermal cycling of the heat exchanger. The use of such a “T-shaped” or “I-shaped” perforation may be difficult to maintain since the perforation may fill up with filler metal such as cladding or solder during the brazing of the core.